1. Field of the Invention
The present invention relates to a high-isolation switching device for a millimeter-wave band control circuit, and more particularly, to a compound semiconductor switching device that is the core device of a switch microwave monolithic integrated circuit (MMIC) used to control transmission and reception of a high-frequency signal in a millimeter-wave band communication system, has high isolation and low insertion loss, and is appropriate for designing and manufacturing a small radio frequency (RF) control circuit chip.
The present invention has been produced from the work supported by the IT R&D program of MIC (Ministry of Information and Communication)/IITA (Institute for Information Technology Advancement) [2006-S-077-01, Components/System for Millimeter-wave Passive Image Sensor] in Korea.
2. Discussion of Related Art
In a high-frequency communication system such as a wireless local area network (LAN), a radar system for car collision avoidance, etc., millimeter-waves in a several tens of GHz band are generally used. And, a switching device for switching such a high-frequency signal is often used in a switching circuit of an antenna, a transmission/reception switching circuit, and so on.
A field effect transistor (FET) such as a high electron mobility transistor (HEMT), a metal-semiconductor field effect transistor (MESFET), etc., is generally used as the switching device. Here, the HEMT is a compound semiconductor transistor that has a fine transmission characteristic and a drive voltage characteristic in a millimeter-wave band, low current consumption, includes a simple bias circuit, and facilitates implementation of multiple ports and integration.
Such a switch circuit needs a technique to minimize insertion loss and reduce isolation deterioration caused by parasitic components such as inductance, capacitance, etc. In particular, for a small RF control circuit, design of a high-isolation switching device is very important.
A single-pole-double-throw (SPDT) switch circuit mainly used for changing transmitting and receiving paths of a signal uses only a shunt structure. This is because it is difficult to obtain an isolation equal to or less than −30 dB between the transmitting path and the receiving path in a series-shunt structure, which has too much insertion loss and does not ensure isolation in a millimeter-wave band of a several tens of GHz.
The shunt structure connects a ground via hole to a drain or source of a switching device, and adjusts the voltage of a gate, which is a control electrode, according to a millimeter-wave signal input to the source or drain electrode, thereby making an unwanted signal flow to ground and finally intercepting the flow to an output end.
Conventionally, a multi-stage shunt technique is generally used to ensure high isolation in such a shunt structure. However, when the multi-stage shunt technique is used, a chip size increases due to a λ/4 transformer transmission line, a plurality of FETs, and an inductor or capacitor added around a switching device, thereby increasing manufacturing cost.
To solve this problem, a “Millimeter-Band Semiconductor Switching Circuit” that improves isolation by minimizing a distance between a via hole and a transmission line is disclosed in U.S. Pat. No. 6,320,476 (filed on Nov. 20, 2001).
However, the switching circuit has a structure in which a transmission line and via holes are perpendicularly connected for minimizing the distance between them. Since only 2 via holes can be disposed, there is a limit to the degree of isolation per unit cell, and insertion loss increases in proportion to the impedance of the transmission line. In addition, like the conventional multi-stage shunt technique, chip manufacturing cost increases.